Wireless communication devices may be referred to as mobile telephones, user equipments (UE), wireless terminals, mobile terminals, mobile stations, cellular telephones, smart phones, laptops, tablets and phablets, i.e. a combination of a smartphone and a tablet with wireless capability. Wireless communication devices are enabled to communicate or operate wirelessly in a wireless communication system comprising multiple networks or Heterogeneous Networks (HetNet) with access nodes or access points. The heterogeneous networks may comprise, e.g. a cellular communications network comprising Second I/Third Generation (2G/3G) network, such as Global System for Mobile Communications (GSM), Wideband Code Division Multiple Access (WCDMA) or High Speed Packet Access (HSPA) etc., 3G Long Term Evolution (LTE) network, Worldwide interoperability for Microwave Access (WiMAX) network, Wireless Local Area Network (WLAN) or WiFi etc. for proving different type of radio access technologies (RATs). A wireless communications network may cover a geographical area which is divided into cells or cover areas, wherein each cell is served by a network node, which may also be referred to as a serving network node, an access node, an access point or a base station, e.g. eNodeB (eNB) or NodeB.
The development of new generations of cellular systems simultaneously with upgrading existing generations allows for a wider range of accessible networks and RATs. In an environment where e.g., both LTE and HSPA co-exist, data rates for the two RATs are comparable. Furthermore, both LTE and HSPA allow for multi carrier signalling. In LTE this capability is denoted as Carrier Aggregation (CA), allowing for up to five LTE carriers to be aggregated, whereas in HSPA it is denoted as Multi Carrier (MC), allowing for up to eight HSPA carriers to be aggregated.
Further, specifications for the Evolved Packet System (EPS), also called a Fourth Generation (4G) network, have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases, for example to specify a Fifth Generation (5G) network. This will provide more accessible networks and RATs.
An arising scenario today is multiple Subscriber Identity Modules (SIMs) devices, such as UEs may carry two or more SIMs from a single or multiple operators in the same device. Particularly in Asia this has become de facto standard, although it has not been standardized by the 3rd Generation Partnership Project (3GPP). On many markets it is hard to get operator approval and volumes for a mid-end device without the capability of supporting at minimum Dual SIM Dual Standby (DSDS). The capability of supporting DSDS allows a UE to camp on two cells simultaneously, or be connected to one cell and camp on another cell. In case both SIMs are from the same operator, the UE may occasionally camp on the same cell but with two different identities and associated paging occasions. In order to qualify for high-end device approval, it is generally required to support Dual SIM Dual Activity (DSDA), whereby the UE can be independently connected towards two cells simultaneously.
The popularity of DSDS/DSDA devices on Asian markets depends on several factors. One factor may be that operators have different price plans e.g. for data and voice, or may have different price plans depending on calling subscribers in same or other network. Other factors may be, e.g. different coverage by different operators, i.e. spotty coverage, or that a mobile phone number cannot move between operators. The trend is towards to support even more than two SIMs simultaneously, and UEs with support for three and four SIMs, Triple SIM, Triple Standby (TSTS) and Quad SIM Quad Standby (QSQS) have been announced by some UE vendors.
For DSDA UEs in active mode, it is required for the UE to use separate receivers for each connection, since it e.g. may use a Packet Switched (PS) service simultaneously for both SIMs, or may use PS service for one and a Circuit Switched (CS) service for the other. Therefore to support DSDS/DSDA, TSTS/QSQS and different RATs, the wireless communication devices usually comprise multiple receiving paths.
Moreover multi-antenna UEs have been introduced in LTE network. Although not explicitly stated, requirements that were defined for Evolved Universal Terrestrial Radio Access (E-UTRA) in the 3GPP LTE specification Release 8 were impossible to pass without two antennas. In parallel, UEs with dual antennas were introduced also in the previous generation of cellular systems, such as in WCDMA.
3GPP's Radio Access Network Working Group 4 (RAN 4) is responsible for defining the requirements for transmission and reception parameters, channel demodulation and radio resource management. Although the functionality of up to eight Multi-input Multi-output (MIMO) streams has been specified in 3GPP RAN 1 since Release 10, it is not until the ongoing Release 13 that the work on the requirements specification was instigated for up to 4 MIMO streams.
Four downlink receive antennas (DL 4 RX) increases the spatial diversity allowing for more parallel streams to be transmitted when conditions allow, or for the UE to being able to cancel out interferers while receiving data allowing for higher modulation orders or code rates although not necessarily using more parallel layers. All in all this will result in significantly higher data rates compared to a standard DL 2 RX UE. Future UEs may very well be equipped with even more antennas, in particular considering the increased carrier frequencies that are being introduced in future standards, allowing for smaller but more antennas on the same space.
MIMO allows multiple layers to be transmitted in parallel streams over the same physical time-frequency resource. The number of parallel streams are, however, limited to the lesser number of the number of transmit antennas and the number of receive antennas. Quite often, in practice the number of streams is even less than that. The actual value depending on several factors such as radio propagation environment, as well as the antenna design of both the eNB and the UE.
For improving the performance of wireless communication devices with multiple SIMs and multiple antennas and receiving paths, it is desirable to handle the operating modes and schedule the connections of the multiple SIMs to the networks. One of present solutions for scheduling between multiple SIMs in e.g. a dual-SIM UE, is to share the existing RX chains or paths. In case one SIM is in active mode, i.e. receiving data or voice call, and the other in idle mode, i.e. only occasionally monitoring paging and doing Radio Resource Management (RRM) measurements. When the SIM in idle mode needs to perform idle mode tasks e.g., paging or mobility management, the SIM in active mode needs to interrupt its signaling to allow for the idle mode tasks to be performed. For a paging operation, the interruption is approximately 4-5 subframes due to necessary Automatic Gain Control (AGC) and Automatic Frequency Control (AFC) calibration being performed on subframes preceding the paging subframe. During this time the SIM in active mode is unable to receive any signal at all, including data, control signaling or Hybrid Automatic Repeat Request (HARQ) retransmissions, which results in degraded performance, e.g. interruptions in voice call, data download etc.